This invention relates to apparatus for surveying radiation. In particular, this invention relates to a circuit for application in a radiation survey meter to correct for counting losses and resulting inaccuracies due to dead time in the detector and the count-rate circuit.
In radiation monitoring, several types of detectors are used to sense the presence of photons and particles that comprise radiation that may be harmful to humans. These detectors include Geiger-Mueller (GM) tubes, scintillator-photomultiplier combinations, and proportional counters. Each of these devices produces an output pulse in response to the passage of a photon or a charged particle in a specified energy range through a measuring region of the detector. The passage of such a photon or particle disables the detector briefly during a period that is referred to as dead time. A particle or photon that passes through the measuring region during the dead time will not produce an output and therefore will not be counted. The actual dead time of a GM tube is typically of the order of tens to hundreds of microseconds, depending upon the tube type, and it varies slightly from tube to tube of a given type. The actual dead time of scintillator-photomultiplier combinations and proportional counters is typically less than that of all but the fastest-recovering GM tubes.
When the count rate is low, the effect of dead time is negligible. However, when the count rate approaches the order of the reciprocal of the dead time, the dead time causes the observed counting rate to be significantly different from the actual counting rate. Thus, in order to provide an indication of the actual counting rate, it is necessary to correct for the dead time.
The problem is formulated mathematically in "Radiological Health Handbook," published by the U.S. Department of Health, Education and Welfare, Bureau of Radiological Health, January 1970, p. 121. It is restated by Todd, U.S. Pat. No. 4,292,539, entitled "Compensated Count-Rate Circuit for Radiation Survey Meter," which is assigned to the assignee of the present invention and which is incorporated here by reference as if set forth fully. Todd observes that, for random inputs, the actual count per unit time (N) is related to the observed count per unit time (n) by the relation ##EQU1## where t.sub.d is the dead time of the system and nt.sub.d is always less than or equal to one.
Various attempts have made in the past to correct for dead time in a rate meter with a logarithmic scale. One method that has been used is to reduce the sensitivity of the detector. This reduces the effect of dead time, but sacrifices precision in measuring low count rates, which are the rates most detected in applications involving health physics. Another means of accounting for dead time that has been used is to determine a correction empirically and apply that correction across the range of values measured. This has the disadvantage that such an empirical correction is typically specific to a particular tube, and must be revised when the tube is changed. A third method that has been used is to add a linear element in series with a log-determining component, which provides a correction that is adequate to about 20% dead time. A fourth that has been applied is the simultaneous switching of detectors and ranges to extend the dynamic range of the instrument beyond two decades. This has the disadvantage of potentially causing operator confusion regarding the scale or scale multiplier in use. Thus, there is a need for an improved circuit to correct for dead time in logarithmic rate meters.